Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
Browse
2 results
Search Results
Item Response of the coastal ocean and estuaries to tropical cyclones(2018) Zhang, Fan; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Landfalling tropical cyclones (TC) pose great threats to public safety. The recent decades have witnessed major advances of knowledge in TC dynamics and improvement in TC forecast models, however, occasionally inaccurate TC intensity and storm surge predictions remain a vital concern. Different representations of subgrid-scale physics by various atmospheric model parameterization schemes lead to uncertainty in predictions of TC’s intensity and associated surges. In a case study for Hurricane Arthur (2014), local closure scheme for planetary boundary layer turbulence produces lower equivalent potential temperature than non-local closure schemes, leading to under-predicted TC intensity and surge heights. On the other hand, higher-class cloud microphysics schemes over-predict TC intensity and surge heights. Without cumulus parameterization for coarse-resolution grids, both TC intensity and surge heights are grossly under-predicted due to large precipitation decreases in the storm center. To avoid widespread predictions, the ensemble mean approach is shown to be effective. Another source of TC forecast error is inaccurate sea surface temperature (SST) prediction, and accurate SST prediction necessitates a better understanding of mixing processes in the coastal ocean. Previously, the importance of TC-induced near-inertial currents (NICs) to mixing in the coastal ocean was overlooked. With high-frequency radar and autonomous glider, long-lasting NICs with amplitudes of ~0.4 m s-1 were observed on the shelf during Arthur. With an atmosphere-ocean model, we find the NICs were dominated by mode-1 vertical structure and were a major contributor to the shear spectrum. Therefore, NICs may be important in producing turbulent mixing and surface cooling during Arthur’s passage. In the future, with warmer SST, sea level rise, and possible hard shorelines in estuaries, increased storm surge hazard is expected. Using Isabel (2003) as a case study, we find storm intensification under 2100 SST raises surge heights in Chesapeake Bay by 0.1-0.4 m given increased energy input. While sea level rise in 2100 reduces surge heights by 0-0.15 m through non-linear processes, it increases total water level by 0.4-1 m. Moreover, hard shoreline further increases surge heights by up to 0.5 m in the middle and upper Chesapeake Bay by prohibiting energy flux towards wetlands.Item Roles of the symbiotic microbial communities associated with sponge hosts in the nitrogen and phosphorus cycles(2015) Zhang, Fan; Hill, Russell T; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Marine sponges are habitat-forming organisms in coral reefs. Many sponge species host highly abundant microorganisms inside their bodies, forming symbiotic relationships. Efficient nutrient cycling between the symbiotic microbial communities and their hosts is considered to be a vital mechanism to retain limited resources inside the holobiont, providing a competitive edge in an environment where ambient nutrient availability is extremely low. In this dissertation, I describe microbially mediated nitrogen fixation, ammonia oxidation and phosphorus accumulation in keystone sponge species, combining culture dependent and independent methods to characterize these functional pathways. Firstly, I characterized the symbiotic diazotrophic communities using nitrogenase gene marker nifH and by culturing representative diazotrophs. I found that various groups of cyanobacteria and heterotrophic bacteria actively express nifH genes during the entire day-night cycle, an indication that the nitrogen fixation potential was fully exploited by different N-fixing bacterial groups associated with their hosts. Archaea associated with marine sponges can actively affect the fate of fixed nitrogen in the holobiont. In order to elucidate the relative importance of host specificity and biogeographic background in shaping the symbiotic archaeal communities, I investigated these communities in sympatric sponges from the Mediterranean and the Caribbean. Based on 16S rRNA and amoA genes, the community structure in M. laxissima differed from that in Ircinia spp., including the sympatric sponge I. strobilina, indicating that host-specific processes control the sponge-archaeal communities. Compared with the nitrogen cycle, the phosphorus cycle has been little studied in sponge microbiology. I found significant accumulation of polyphosphate (polyP) granules in three common sponge species from Caribbean coral reefs. The identity of the polyP granules was confirmed by energy-dispersive spectroscopy and by fluorescence properties of the granules. Microscopic images revealed that a large proportion of microbial cells associated with sponge hosts contained intracellular polyP granules. Based on these findings, I propose a potentially important phosphorus sequestration pathway through symbiotic microorganisms of marine sponges. Considering the widespread sponge population and abundant microbial cells associated with them, these pathways are likely to have a significant impact on the nitrogen and phosphorus cycle in benthic coral reef ecosystems.